Method and apparatus for cooling a CVI/CVD furnace

ABSTRACT

The invention relates to method and apparatus for cooling a furnace configured for processing refractory composites. More specifically, the invention is directed to method and apparatus for cooling a furnace more rapidly than prior art methods. According to the invention, a cooling gas is flowed in a closed circuit through the furnace, over the refractory composites disposed within the furnace, and over a cooling element disposed within the furnace. The cooling gas may be flowed by natural convection or by force.

BACKGROUND

[0001] The invention relates to method and apparatus for cooling afurnace configured for processing refractory composites. Morespecifically, the invention is directed to method and apparatus forcooling a furnace more rapidly than prior art methods.

[0002] Processing of refractory composites takes place at elevatedtemperatures. Such processing includes CVI/CVD deposition of a bindingmatrix within a fibrous preform structure, and heat treating refractorycomposites. According to prior practice, the furnace is allowed to coolstatically under vacuum or back-filled with an inert gas such asnitrogen. Cooling the furnace to a low enough temperature wherein thefurnace may be opened can take days according to this practice. Inaddition, cooling the furnace too rapidly or introducing a reactive gas,such as oxygen, can cause damage to the furnace or the refractorycomposites being processed in the furnace. Therefore, a method andapparatus is desired whereby the furnace and the refractory compositesare cooled more rapidly and at a controlled pace without damage.

SUMMARY OF THE INVENTION

[0003] According to an aspect of the invention, a method is provided forcooling a furnace configured to process refractory composites,comprising the steps of: flowing a cooling gas in a closed circuitthrough the furnace, over the refractory composites disposed within thefurnace, and over a cooling element disposed within the furnace. Themethod according to the invention may further comprise the step offlowing the cooling gas by natural convection. The method according tothe invention may also further comprising the step of flowing thecooling gas by forced flow.

[0004] According to a further aspect of the invention, a furnaceconfigured to process refractory composites and a cooling systemtherefor is provided, comprising: a furnace shell that defines a furnacevolume; a heater disposed within the furnace shell; a cooling elementdisposed within the furnace shell; an inlet conduit connected to thefurnace shell in fluid communication with the furnace volume; an outletconduit connected to the furnace shell in fluid communication with thefurnace volume; a cooling gas supply configured to selectively introducea cooling gas into the furnace volume; and, a blower connected to theinlet conduit and the outlet conduit in fluid communication therewith,wherein activation of the blower causes cooling gas introduced into thefurnace volume to flow through the blower, through the inlet conduit,over the cooling element, through the outlet conduit, and back to theblower in a closed circuit.

[0005] The invention includes various other aspects as presented by thedetailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 presents a schematic view of a cooled furnace according toan aspect of the invention wherein the flow of cooling gas is induced bynatural convection.

[0007]FIG. 2 presents a schematic view of a cooled furnace according toan aspect of the invention wherein the cooling element is the furnaceshell and the induction coil.

[0008]FIG. 3 presents a schematic view of a cooled furnace according toan aspect of the invention wherein the flow of cooling gas is forced.

[0009]FIG. 4 presents a schematic view of a cooled furnace according toan aspect of the invention wherein the flow of cooling gas is forced andwherein the inlet and outlet conduits are in an alternate position.

[0010]FIG. 5 presents a schematic view of a cooled furnace according toan aspect of the invention wherein the flow of cooling gas is forced andwherein the inlet and outlet conduits are in an alternate position.

[0011]FIG. 6 presents a schematic view of a cooled furnace according toan aspect of the invention wherein the flow of cooling gas is forced andwherein the reactive gas inlets are implemented to introduce a flow ofcooling gas.

[0012]FIG. 7 presents an embodiment of the invention wherein cooling gasis introduced at multiple locations including the reactive gas inlets,and the cooling element is the furnace shell and the induction coil.

[0013]FIG. 8 presents a cross-sectional view of a blower with an inertgas purged dynamic shaft seal, according to an aspect of the invention.

[0014]FIG. 9 presents a side cross-sectional view of a furnace accordingto a certain embodiment of the invention.

[0015]FIG. 10 presents a side cross-sectional view of a furnaceaccording to a certain embodiment of the invention.

DETAILED DESCRIPTION

[0016] Various aspects of the invention are presented in FIGS. 1-10,which are not drawn to scale, and wherein like components are numberedalike. Referring now to FIGS. 1-6, schematic representations of basicconcepts according to certain aspects of the invention are presented.Literal interpretation of the configurations presented in FIGS. 1-6 isnot intended since the actual configuration may vary greatly dependingupon the particular implementation into a specific furnace design.Specifically referring now to FIG. 1, method and apparatus are presentedfor cooling a furnace 90 configured to process refractory composites,comprising the step of flowing a cooling gas 106 in a closed circuitthrough the furnace 90 and over a cooling element 104 disposed withinthe furnace 90, as indicated by the flow path 94. The gas is also flowedover refractory composites 62 disposed inside the furnace 100. As usedherein, the term “refractory composites” includes fibrous refractoryarticles fully or partially permeated with a binding refractory matrix,and intermediate refractory articles (refractory fibrous preformstructures, for example, such as carbon or ceramic fiber brake diskpreforms). A cooling medium, such as water, is circulated through thecooling element 104 and a heat exchanger 105 external to the furnace100. The furnace 90 comprises a furnace shell 92 that defines a furnacevolume 114, and is disposed upon legs 113. A heater 116 is also disposedwithin the furnace 90 and heats the refractory composites 62 for CVI/CVDand/or heat treatment processing. The gas flow is driven by naturalconvection. The refractory composites are quite hot at the beginning ofthe cooling process and heat the cooling gas to an elevated temperaturewhich causes it to rise where it is cooled by the cooling element 104.The cooled gas falls due to the force of gravity and is directed towardthe outer circumference of the furnace and back up through the bottom.The cooling gas 106 is supplied to the furnace volume 114 by a coolinggas supply 122 that may be comprised of a single gas or a plurality ofindividual gas supplies 123 with individual flow quantities becontrolled by flow control valves 125.

[0017] Referring now to FIG. 2, a furnace 96 having a furnace shell 98is presented according to similar arrangement wherein natural convectionprovides the motive force for the cooling gas 106, as indicated by theflow path 99. In furnace 96 the heating element 116 comprises asusceptor 158 and an induction coil 160 disposed adjacent the susceptor158, and the cooling element is configured to cool the furnace shell 98,which, in this example, comprises a double wall with a space 97 inbetween filled with cooling water that is circulated through the heatexchanger 105. The space 97 may be separated into multiple sub-spaceswith independent cooling water flow circuits. In addition, inductioncoils typically comprise a multitude of coil cooling passages 162integrally formed into the induction coil 160. Thus, the cooling elementmay further comprise the induction coil 160 with integral coolingpassages 162 although, according to a preferred embodiment, the coolingis predominantly (if not totally) provided by the shell 146. In theembodiments of FIG. 1 and 2, openings may be provided through thevarious components and fixtures within the furnace may be provided toallow the cooling gas to flow in the manner described, or externalconduits may attached to the outside of the furnace to provide part ofthe flow path outside the furnace.

[0018] Flow of the cooling gas 106 through the furnace may also beforced. Referring now to FIG. 3, a method is presented for cooling afurnace 100 configured to process refractory composites, according to afurther aspect of the invention, comprising the step of flowing acooling gas 106 in a closed circuit 102 through the furnace 100 and overa cooling element 104 disposed within the furnace 100. The gas is alsoflowed over refractory composites 62 disposed inside the furnace 100. Acooling medium, such as water, is circulated through the cooling element104 and a heat exchanger 105 external to the furnace 100. According to afurther aspect of the invention, a method is provided for cooling thefurnace 100, comprising the step of flowing the cooling gas 106 throughthe closed circuit 102 and over a cooling element disposed within thefurnace, the closed circuit including the furnace 100, and a blower 108disposed outside the furnace 100. The methods according to the inventionmay further comprise the step of monitoring the oxygen content of thecooling gas 106. An oxygen content analyzer 110 may be provided thatsenses the oxygen content of the cooling gas 106 in the closed circuit102. The oxygen content is preferably maintained below a predeterminedvalue. For most processes, the oxygen content should be less than orequal to 100 ppm.

[0019] The invention is particularly useful for cooling furnaces used inhigh temperature CVI/CVD and/or heat treatment processes. As usedherein, the term “high temperature” means a temperature substantiallyelevated above room temperature in the range of 300 C. or greater.Refractory materials, generally, are manufactured and/or processed attemperatures greater than 300 C., and may be at least 900 C. and on theorder of 900-3000 C., or higher. For example, a porous carbon aircraftbrake disk may have a pyrolytic carbon matrix deposited within it by aCVI/CVD process conducted at a temperature in the range of 900-1100 C.,and may be heat-treated at a temperature up to 2200 C. or higher.Manufacturing and processing of other types of ceramic materials mayoccur at other temperatures.

[0020] Still referring to FIG. 3, a furnace and cooling system arepresented that may be implemented in practicing the invention. Accordingto an aspect of he invention, a combination is provided comprising thefurnace 100, a cooling gas inlet 118 in fluid communication with thefurnace 100, a cooling gas out let 120 in fluid communication with theCVICVD furnace 100, a cooling element 104 disposed within the furnace100 in a location where it may be exposed to cooling gas 106, and ablower 108 outside the furnace 100 connected to the cooling gas inlet118 and the cooling gas outlet 120, wherein the blower 108 causescooling gas to flow in a closed circuit 102 through the cooling gasinlet 118, through the furnace 100 over the cooling element 104, andthrough the cooling gas outlet 120 back to the blower 108. According toa further aspect of the invention, the furnace 100 comprises a furnaceshell 112 that defines a furnace volume 114. The furnace shell may bedisposed upon legs 113. A heater 116 and the cooling element 104 aredisposed within the furnace shell 112. The cooling gas inlet 118 may beformed as an inlet conduit connected to the furnace shell 112 in fluidcommunication with the furnace volume 114. The cooling gas outlet 120may be formed as an outlet conduit also connected to the furnace shell112 in fluid communication with the furnace volume 114. A cooling gassupply 122 configured to selectively introduce the cooling gas 106 intothe furnace volume 114. The blower 108 is connected to the inlet conduit118 and the outlet conduit 120 in fluid communication therewith.Activation of the blower causes cooling gas 106 introduced into thefurnace volume 114 to flow through the blower 108, through the inletconduit 118, over the cooling element 108, through the outlet conduit120, and back to the blower 108 in a closed circuit. Although thecooling element 104 is shown at the top of the furnace in FIGS. 14, thisposition may not be the optimum position due to the relatively hightemperatures typically encountered in that region. As will becomeapparent, the cooling element 104 may be placed in a variety ofpositions within the furnace, and the inlet and outlet conduit positionschanged accordingly to cause the cooling gas to flow over the coolingelement 104. In addition, the inlets and outlets may be connected atmultiple locations, as desired, to achieve a particular flow pattern.

[0021] The refractory composites 62 may comprise a multitude of poroussubstrates 62 stacked within the furnace 100 that are heated and exposedto a reactant gas that breaks down and deposits a matrix with the poroussubstrates 62. This process is commonly known as chemical vaporinfiltration and deposition. Chemical vapor infiltration and deposition(CVI/CVD) is a well known process for depositing a binding matrix withina porous structure. The term “chemical vapor deposition” (CVD) generallyimplies deposition of a surface coating, but the term is also used torefer to infiltration and deposition of a matrix within a porousstructure. As used herein, the term CVI/CVD is intended to refer toinfiltration and deposition of a matrix within a porous structure. Thetechnique is particularly suitable for fabricating high temperaturestructural composites by depositing a carbonaceous or ceramic matrixwithin a carbonaceous or ceramic porous structure resulting in veryuseful structures such as carbon/carbon aircraft brake disks, andceramic combustor or turbine components. The generally known CVI/CVDprocesses may be classified into four general categories: isothermal,thermal gradient, pressure gradient, and pulsed flow. See W. V.Kotlensky, Deposition of Pyrolvtic Carbon in Porous Solids, 8 Chemistryand Physics of Carbon, 173, 190-203 (1973); W. J. Lackey, Review,Status, and Future of the Chemical Vapor Infiltration Process forFabrication of Fiber-Reinforced Ceramic Composites, Ceram. Eng. Sci.Proc. 10[7-8] 577, 577-81 (1989) (W. J. Lackey refers to the pressuregradient process as “isothermal forced flow”). In an isothermal CVI/CVDprocess, a reactant gas passes around a heated porous structure atabsolute pressures as low as a few torr. The gas diffuses into theporous structure driven by concentration gradients and cracks to deposita binding matrix. This process is also known as “conventional” CVIICVD.The porous structure is heated to a more or less uniform temperature,hence the term “isothermal.” In a thermal gradient CVI/CVD process, aporous structure is heated in a manner that generates steep thermalgradients that induce deposition in a desired portion of the porousstructure. The thermal gradients may be induced by heating only onesurface of a porous structure, for example by placing a porous structuresurface against a susceptor wall, and may be enhanced by cooling anopposing surface, for example by placing the opposing surface of theporous structure against a liquid cooled wall. Deposition of the bindingmatrix progresses from the hot surface to the cold surface. In apressure gradient CVI/CVD process, the reactant gas is forced to flowthrough the porous structure by inducing a pressure gradient from onesurface of the porous structure to an opposing surface of the porousstructure. Flow rate of the reactant gas is greatly increased relativeto the isothermal and thermal gradient processes which results inincreased deposition rate of the binding matrix. This process is alsoknown as “forced-flow” CVI/CVD. Finally, pulsed flow involves rapidlyand cyclically filling and evacuating a chamber containing the heatedporous structure with the reactant gas. The cyclical action forces thereactant gas to infiltrate the porous structure and also forces removalof the cracked reactant gas by-products from the porous structure.Refractory composites are often subjected to heat treatments at varioustemperatures, and the invention is equally useful in furnaces employedfor that purpose. The furnace and fixture configuration may varysubstantially depending upon the type of process, and the variousaspects of the invention may be implemented with any of these processes,depending upon the particular configuration. As such, the furnaceconfiguration of FIGS. 1-9 is presented by way of example, and is notintended to limit the invention to the specific arrangements presentedas other variations are evident to persons skilled in the art in lightof the description provided herein.

[0022] According to a certain embodiment, the cooling gas 106 comprisesa predetermined ratio of gasses. The cooling gas supply 122 may comprisea multitude of individual gas supplies 123 in fluid communication withthe inlet conduit 118. Each individual gas supply 123 may provide adifferent gas composition, and flow control valves 125 may be providedto control flow of a particular gas composition into the inlet conduit118. The flow control valves 125 may be used in combination to provide aflow of gas into the inlet conduit 118 comprising a predetermined ratioof gasses by individually controlling the flow of each gas. The gassupply 122 may be connected to the furnace 100 in other ways thatintroduce the flow of cooling gas into the furnace, for example, byconnecting the gas supply 122 directly to the furnace 100, or byconnecting the gas supply 122 to the outlet 120. Other alternatives forparticular applications are apparent to a person of ordinary skill inthe art in light of the description provided herein. The individual gassupplies 123 may be bottles of gas or a gas supply otherwise availableat the manufacturing facility, a plant nitrogen supply for example.Other suitable gasses for cooling include helium and argon, typicallysupplied by bottle. Nitrogen is relatively inexpensive, but may reactwith materials inside the furnace at elevated temperatures. For example,nitrogen may react with carbon/graphite above 2500° F. to form cyanogengas. Helium has a higher thermal conductivity than nitrogen or argon,but has a lesser atomic weight than nitrogen or argon so more isrequired. Argon is more stable than nitrogen at elevated temperatures,especially above 2500° F., has a much greater atomic weight than helium,and has a greater heat capacity than helium or nitrogen. An idealmixture takes advantage of all of these characteristics to provide theleast expensive mixture with optimum cooling characteristics at thetemperatures encountered for a particular process. The optimum mixturemay be different for different processes and depends upon the peaktemperatures encountered.

[0023] Alternatively, a single cooling gas such as nitrogen may beemployed. If the cooling gas is reactive at a certain criticaltemperature, back-filling the furnace volume with the cooling gas may bedelayed while the furnace cools under vacuum to a temperature less thanthe critical temperature according to prior practice in the art. Thecooling gas is subsequently introduced into the furnace volume andcirculated in the manner described. For example, if nitrogen is used asthe cooling gas, the furnace may be allowed to cool under vacuumaccording to prior practice in the art until reactive components are ata temperature on the order of 2000° F. or less, after which the furnacevolume is filled with the cooling gas to approximately atmosphericpressure and the cooling gas is circulated. The furnace volume may bepartially filled if the temperature is greater than the criticaltemperature, which may increase the cooling rate with minimal chemicalreaction. The temperature at which certain cooling gasses are introducedmay be dependent upon the reactivity of certain components within thefurnace. The presence of certain cooling gasses and the overallcomposition of the cooling gas may be altered accordingly.

[0024] The composition of the cooling gas may be changed while it isbeing circulated in order to effect the rate at which the furnace iscooled. For example, the cooling rate typically decreases if the coolingconditions are not changed. Changing the cooling conditions may increaseor decrease the rate as a function of time. According to a certainembodiment, the composition of the cooling gas is changed to produce aconstant rate at which the furnace is cooled, which produces anapproximately linear time versus temperature curve (negative constantslope). The flow rate of the cooling gas may also be altered to effectthe rate at which the furnace is cooled, for example by increasing ordecreasing the rate. According to a certain embodiment, the flow rate ofthe cooling gas is altered to produce a constant rate at which thefurnace is cooled, which produces an approximately linear time versustemperature curve (negative constant slope). According to a preferredembodiment, both the gas composition and the cooling gas flow rate arechanged during the cooling process to produce a constant rate at whichthe furnace is cooled and a linear time versus temperature curve.

[0025] In the embodiment presented in FIG. 3, the furnace shell has twoend portions 130 and 132, and the inlet conduit 118 is connected to oneof the end portion 130. The position of the inlet conduit 118 and outletconduit 120 depends, in part, upon the desired flow pattern of coolinggas through the furnace volume 114. As such, innumerable variations arepossible. Referring now to FIG. 2, for example, a CVI/CVD furnace andcooling system is presented wherein the position of the inlet conduit118 is moved to produce a change in the flow of the cooling gas. Thevarious components previously described in relation to FIG. 3 arepresented in FIG. 4, except that the furnace 100 is replaced by afurnace 124, having a furnace shell 126 with a center portion 128disposed between two end portions 130 and 132. According to this aspectof the invention, a closed circuit 134 having an inlet conduit 136 isconnected to the furnace 100 at the center portion 128. Connecting theinlet conduit 102 to the furnace 100 at the center portion 128 providesa flow of the cooling gas to the area that is typically the hottest.

[0026] Referring now to FIG. 5, a CVIICVD or heat treatment furnace andcooling system is presented that combines the features of FIGS. 3 and 4.The various components previously described in relation to FIGS. 1 and 2are presented in FIG. 5, except that furnace 138 having a furnace shell140 is provided. The furnace shell 140 has a center portion 128 disposedbetween two end portions 130 and 132. According to this aspect of theinvention, a closed circuit 142 having inlet conduits 118 and 136 isconnected to the furnace 138 at the end portion 130 and the centerportion 128, respectively. Connecting the inlet conduit 136 to thefurnace 138 at the center portion 128 provides a flow of the cooling gasto an area of the furnace 138 that is typically the hoftest, whileconnecting the inlet conduit 118 to the furnace 138 at the end portion130 provides a flow of gas to substrates 62 disposed below the inletconduit 136. Multiple inlet conduits 136 may be provided. The outletconduit 120 is connected to the other of the end portions 132. Overallcooling of the furnace may thereby be improved relative to theembodiments of FIGS. 3 and 4.

[0027] Other connections into a furnace may also be utilized as coolinggas inlets or cooling gas outlets. Referring now to FIG. 6, for example,a CVI/CVD or heat treatment furnace and cooling system is presentedaccording to a further aspect of the invention. The various componentspreviously described in relation to FIG. 5 are presented in FIG. 6. Afurnace 144 is provided having a furnace shell 146 with a center portion128 disposed between two end portions 130 and 132. Furnace 144 comprisesa reactant gas inlet 148 connected to the furnace shell 146 in fluidcommunication with the furnace volume 114. A closed circuit 152 isprovided wherein the inlet conduit 118 is connected to the furnace shell146 through the reactant gas inlet 148 and is configured to selectivelyintroduce cooling gas into the furnace volume 114 through the reactantgas inlet 148. Thus, the inlet conduit 118 is in fluid communicationwith the furnace volume 114 through the reactant gas inlet 148. Attimes, reactant gas flow rather than cooling gas flow is desired throughthe reactant gas inlet 148. Thus, cooling gas is selectively introducedinto the furnace volume 114 when such flow is desired. This ispreferably accomplished by provision of a valve 150 provided in theinlet conduit 118 that isolates the reactant gas inlet 148 from theinlet conduit 118 when closed. The inlet conduit 136 may be provided andconnected to the center portion 128.

[0028] Referring now to FIG. 7, a preferred embodiment of the inventioncomprising a furnace 154 and a closed circuit 156. In furnace 154 theheating element 116 comprises a susceptor 158 and an induction coil 160disposed adjacent the susceptor 158, and the cooling element isconfigured to cool the furnace shell 146, which, in this example,comprises a double wall with a space 147 in between filled with coolingwater that is circulated through the heat exchanger 105. The space 147may be separated into multiple subspaces with independent cooling waterflow circuits. In addition, induction coils typically comprise amultitude of coil cooling passages 162 integrally formed into theinduction coil 160. Thus, the cooling element may further comprise theinduction coil 160 with integral cooling passages 162 although,according to a preferred embodiment, the cooling is predominantly (ifnot totally) provided by the shell 146.

[0029] The susceptor 158 typically comprises a susceptor lid 164 and asusceptor floor 166. The reactant gas inlet 148 passes through thesusceptor floor 166. The outlet conduit 120 is disposed beneath thecenter portion 128, and the inlet conduit 136 is connected to the centerportion 128 and passes through the induction coil 160 and susceptor 158.Cooling gas introduced into the inlet conduits 118 and 136 enters thevolume encircled by the susceptor 158 where the porous substrates 62 aredisposed. The cooling gas then passes up through the susceptor lid 164(which is typically perforated) and over the inside surface of thefurnace shell 146 and down between the furnace shell 146 and theinduction coil 160, where it is cooled, and then passes into the outletconduit 120 and back to the blower 108. Activation of the blower 108causes cooling gas 106 introduced into the furnace volume 114 to flowthrough the blower 108, through the inlet conduit 118, over the coolingelement (in this example, the shell 146 and induction coil 160 withcooling passages 162), through the outlet conduit 120, and back to theblower 108 in a closed circuit. In this embodiment, the cooling elementis embodied in two sub-elements and serves two purposes. It cools theshell 146 and the induction coil 160 when the coil is heating thesusceptor 158 and, alternatively, cools the cooling gas when the closedcircuit 156 is operated to cool the furnace 154.

[0030] Although described in relation to the cooling element being thefurnace shell 146 and/or the induction coil 160 with coil coolingpassages 162, any arrangement disposed within the furnace for thepurpose of cooling a component inside the furnace may be employed tocool the cooling gas, and such arrangements may take a variety ofconfigurations whether employed to cool the furnace shell, an inductioncoil, or otherwise, any of which are intended to be included within thescope of the invention. Finally, the cooling gas inlet may comprise oneor more auxiliary inlets, such as inlet 168 (shown as a dashed line)connected to the furnace above the center portion 128 in order toprovide a flow of cooler gas to the top of the induction coil 160 wherehotter gas from inside the susceptor passes over the induction coil 160in transit to the cooling gas outlet 120 disposed below the centerportion. Other variations may be employed, as desired, to achieve aparticular desired flow pattern and/or to eliminate hot and/or coldspots. Shut-off valves 190 are preferably provided in the auxiliaryinlet 168, the inlet conduit 136, and the outlet conduit 120 thatisolate the furnace 154 from the rest of the closed circuit during aCVI/CVD or heat treatment process. A shut-off valve 192 is preferablyprovided in the reactant gas inlet 148 that isolates the reactant gassupply from the closed circuit 156 while using the closed circuit tocool the furnace 154.

[0031] Referring now to FIG. 8, a cross sectional view of an embodimentof the blower 108 is presented, according to a preferred aspect of theinvention, taken along line 6-6of FIG. 1. The blower 108 comprises ahousing 170 and a drive shaft 172 extending therefrom, and an inert gaspurged dynamic seal 174 between the housing 170 and the drive shaft 172.The housing 170 comprises a main housing 184. A pair of bearingassemblies 186 mounted to the housing 184 support the drive shaft 172.An impeller 188 is attached to the drive shaft 172. The impeller 188 maybe configured for axial flow, centrifugal flow, or a combinationthereof, as a fan or otherwise. The inert gas purged dynamic seal 174comprises a pair of seals 176 that may be spaced apart and disposedwithin a sealed seal housing 182 that is sealed to housing 170, and aninert gas inlet 178 that introduces inert gas 184 into the space betweenthe seals 180 at a pressure greater than atmospheric pressure. Thecooling gas 106 may be employed as the inert gas 184. Purging the spacebetween the bearings with pressurized inert gas eliminates oxygeningress into the cooling gas within the blower 108 and the closedcircuit through which the blower 108 drives cooling gas. An inert gaspurged dynamic seal 174 may not be necessary or desirable in all aspectsof the invention. Other components, such as view ports, may be inert gassealed with dynamic or static seals, depending on whether moving partsare employed. According to a preferred embodiment of the invention forprocessing high temperature composite materials, the entire closedcircuit is sealed to prevent ingress of oxygen into the closed circuit.Carbon seals have been found to be particularly desired for seals 176 insuch an embodiment. Inert gas purged seals may be employed to minimizeor eliminate ingress of oxygen, when desired.

[0032] Referring now to FIG. 9, a cross-sectional view of a hightemperature furnace 10 is presented, by way of example, that implementsvarious aspects of the invention. Furnace 10 is configured to beemployed with a high temperature process. Furnace 10 is generallycylindrical and comprises a steel shell 12 and a steel lid 14 bothformed as double walls with a space 13 in between for circulation ofcooling water, as previously described in relation to FIG. 5. Stillreferring to FIG. 9, the shell 12 comprises a flange 16 and the lid 14comprises a mating flange 18 that seals against flange 16 when the lid14 is installed upon the shell 12. The shell 12 and lid 14 togetherdefine a furnace volume 22 that is reduced to vacuum pressure by a steamvacuum generator (not shown) in fluid communication with the vacuum port20. The shell 12 rests upon a multitude of legs 62. The furnace 10 alsocomprises a cylindrical induction coil 24 adjacent a cylindricalsusceptor 26. The induction coil 24 comprises coiled conductors 23encapsulated by electrical insulation 27. During operation, theinduction coil 24 develops an electromagnetic field that couples withthe susceptor 26 and generates heat within the susceptor 26. Theinduction coil 24 may be cooled, typically by integral water passages 25within the coil 24. The susceptor 26 rests upon a susceptor floor 28 andis covered by a susceptor lid 30. A cylindrical insulation wall 32 isdisposed in between the susceptor 26 and the induction coil 24. Lidinsulation layer 34 and floor insulation layer 36 are disposed over thesusceptor lid 30 and beneath the susceptor floor 28, respectively. Thesusceptor floor 28 rests upon the insulation layer 36 which, in turn,rests upon a furnace floor 38. The furnace floor 38 is attached to theshell 12 by a floor support structure 40 that comprises a multitude ofvertical web structures 42. A reactant gas is supplied to the furnace 10by a main gas supply line 44. A multitude of individual gas supply lines46 are connected in fluid communication with a multitude of gas ports 48that pass through the furnace shell 12. A multitude of flexible gassupply lines 50 are connected in fluid communication with the gas ports48 and a multitude of gas inlets 52 that pass through holes 54 in thefurnace floor 38, the floor insulation layer 36, and the susceptor floor28. A gas preheater 56 rests on the susceptor floor 28 and comprises amultitude of stacked perforated plates 58 that are spaced from other bya spacing structure 60. Each plate 58 is provided with an array ofperforations that are horizontally shifted from the array ofperforations of the adjacent plate 58. This causes the reactant gas topass back and forth through the plates, which diffuses the reactant gaswithin the preheater 56 and increases convective heat transfer to thegas from the perforated plates 58. A multitude of porous substrates 62,for example brake disks, are stacked within the furnace 10 inside thesusceptor 26 on fixtures (not shown for clarity). Suitable fixtures arewell known in the art.

[0033] Still referring to FIG. 9, the susceptor 26 is configured as acylindrical wall 26 having a center portion 66 disposed between two endportions 68 and 70. An inlet conduit 72 enters the furnace 10. Thecenter portion 66 has a hole 74 therein with the inlet conduit 72entering the hole 74 and being configured to introduce cooling gaswithin the cylindrical wall 26 at the center portion 74. An insulatingbushing 76 may be disposed within the hole 74 mating with thecylindrical wall 26 and the inlet conduit 72. In passing through thehole 74, the inlet conduit 72 extends through the induction coil 24 andthe insulation wall 32. The inlet conduit 72 is preferably made from aninsulating material and mates with a steel conduit 73 that is welded tothe furnace at 78. A pliant gasket 80 is disposed between the inletconduit 72 and the steel conduit 73, which permits the inlet conduit 72to move relative to the steel conduit 73 as the furnace 10 heats up andcools down while maintaining a seal. If the bushing 76 is made from aporous insulating material, a bushing seal layer 82 may be bonded thesurface that would otherwise be exposed to reactant gas. The insidediameter of the inlet tube 72 is preferably covered with an impervioussheet if the tube 72 is made from a porous insulating material.According to a preferred embodiment for CVI/CVD depositing a pyrolyticcarbon matrix within carbon fiber porous structures for aircraft brakedisks, the furnace 154 of FIG. 7 is configured as furnace 100 of FIG. 9,preferably with the auxiliary inlet 168. According to a certainembodiment, the inlet conduit 72 is manufactured from porous carbon,such as Porous Carbon 60 material, available from UCAR Carbon CompanyInc., United States of America. The bushing 76 is a rigid felt, such asCalcarb CBCF material, available from Calcarb, Ltd., Scotland, orFibergraph® material, available from SIGRI Polycarbon, Inc., UnitedStates of America. The bushing seal layer 82, pliant gasket 80, andimpervious layer lining inside the inlet conduit 72 are a graphite foil,such as Grafoil® material, also available from UCAR Carbon Company Inc.Calgraph® brand graphite foil may also be employed, also available fromSIGRI Polycarbon, Inc.

[0034] A method of cooling a furnace initially at CVI/CVD processtemperatures (on the order of 1800° F.) proceeds as follows. Valve 192is closed and the volume 22 inside the furnace is back-filled fromvacuum (about 10 torr) to atmospheric pressure by flowing on the orderof 275 SCFH nitrogen, 200 SCFH helium, and 75 SCFH argon. When thepressure of volume 22 reaches on the order of atmospheric pressure, allgas flows are terminated and the valves 190 are opened. The oxygensensor 110 (FIG. 1) is activated along with the fan shaft seal purge.The blower 108 at a speed of 25 Hz (the blower is rated at 800 CFM at 60Hz) is activated and the oxygen level of the cooling gas 106 ismonitored and maintained at less than or equal to 100 ppm. Oxygen levelstypically remain steady in the range of 40-100 ppm, and should reachthat range after 15-30 minutes. Upon temperature inside the furnacedecreasing to on the order of 1050° F. the fan speed is increased to 30Hz and a flow of 30 SCFH helium is initiated and subsequently terminatedafter a period of approximately six hours (the vessel is pressurerelieved to avoid positive pressure above atmospheric). Upon temperaturereaching 750° F. fan speed is increased to 35 Hz and a flow of 30 SCFHof helium is again initiated for another period of approximately sixhours and thereafter terminated. Upon the greatest temperature measuredinside furnace being decreased to a final temperature of 600° F. orless, the furnace lid may be removed and the cooling system deactivated.Alternatively, the cooling system may be left running in order tocirculate atmospheric air through the furnace. Increasing fan speed andhelium flow rate as the furnace cools increases the cooling rate andallows approximation of a linear cool-down (rather than asymptotic) fromthe initial temperature to the final temperature. This method isparticularly useful for cooling carbon/carbon composite brake disks fromCVI/CVD processing temperature.

[0035] A method of cooling a furnace initially at a refractory compositeheat treatment temperature (on the order of 3400° F.) proceeds similarlyto the process just described with the following exceptions. The furnaceis back-filled with a gas mixture that is {fraction (3/4)} argon and{fraction (1/4)} helium since these gasses are stable at that initialtemperature. Less helium is used at greater temperatures in order toprevent cooling at too fast rate, which may damage components inside thefurnace, for example the induction coil and/or the refractory compositestructures being heat treated. When the furnace temperature reaches onthe order of 1850° F., a 30 SCFH flow of helium is initiated.Subsequently, additional helium and higher fan speeds are enacted aspreviously described.

[0036] Referring now to FIG. 10, a furnace 10 according to a furtheraspect of the invention is presented that is similar to furnace 10 ofFIG. 9 except the cooling gas inlet through the side of the furnace isreplaced by a similar inlet that enters the furnace from the bottom andpasses up through the center of the preheater 56. The inlet 72 may besplit into multiple inlets if desired. This furnace may be implementedaccording to the embodiment of FIG. 5.

[0037] It is evident that many variations are possible without departingfrom the true scope and spirit of the invention as defined by the claimsthat follow.

What is claimed is:
 1. A method for cooling a furnace configured toprocess refractory composites, comprising the steps of: flowing acooling gas in a closed circuit through said furnace, over saidrefractory composites disposed within said furnace, and over a coolingelement disposed within said furnace.
 2. The method of claim 1, furthercomprising the step of flowing said cooling gas by natural convection.3. The method of claim 1, further comprising the step of flowing saidcooling gas by forced flow.
 4. A method for cooling a furnace configuredto process refractory composites, comprising the steps of: flowing acooling gas comprising a predetermined ratio of gasses in a closedcircuit through said furnace, over refractory composites disposed withinsaid furnace, and over a cooling element disposed within said furnace.5. A method for cooling a furnace configured to process refractorycomposites, comprising the steps of: flowing a cooling gas through aclosed circuit, over said refractory composites disposed within saidfurnace, and over a cooling element disposed within said furnace, saidclosed circuit including said furnace and a blower disposed outside saidfurnace.
 6. The methods of claims 1, 4, or 5, further comprising thestep of monitoring the oxygen content of said cooling gas.
 7. The methodof claim 4, further comprising the step of maintaining said oxygencontent of said cooling gas is less than or equal to 100 ppm.
 8. Afurnace configured to process refractory composites and a cooling systemtherefor, comprising: a furnace shell that defines a furnace volume; aheater disposed within said furnace shell; a cooling element disposedwithin said furnace shell; an inlet conduit connected to said furnaceshell in fluid communication with said furnace volume; an outlet conduitconnected to said furnace shell in fluid communication with said furnacevolume; a cooling gas supply configured to selectively introduce acooling gas into said furnace volume; and, a blower connected to saidinlet conduit and said outlet conduit in fluid communication therewith,wherein activation of said blower causes cooling gas introduced intosaid furnace volume to flow through said blower, through said inletconduit, over said cooling element, through said outlet conduit, andback to said blower in a closed circuit.
 9. The apparatus of claim 8,wherein said furnace shell has an end portion, said inlet conduit beingconnected to said end portion.
 10. The apparatus of claim 8, whereinsaid furnace shell has a center portion, said inlet conduit beingconnected to said furnace at said center portion.
 11. The apparatus ofclaim 8, wherein said furnace shell has an a center portion disposedbetween two end portions, said inlet conduit being connected to saidcenter portion and one of said end portions, said outlet conduit beingconnected to the other of said end portions.
 12. The apparatus of claim8, wherein said cooling gas supply comprises a plurality of gassesconfigured to be supplied in a predetermined ratio.
 13. The apparatus ofclaim 8, further comprising a reactant gas inlet connected to saidfurnace shell in fluid communication with said furnace volume, saidinlet conduit being connected to furnace shell through said reactant gasinlet and configured to selectively introduce cooling gas into saidfurnace volume through said reactant gas inlet.
 14. The apparatus ofclaim 8, wherein said heating element comprises a susceptor and aninduction coil disposed adjacent said susceptor, and said coolingelement is configured to cool said induction coil.
 15. The apparatus ofclaim 8, wherein said blower comprises a housing and a drive shaftextending therefrom, and an inert gas purged dynamic seal between saidhousing and said drive shaft.
 16. A furnace configured to processrefractory composites and a cooling system, comprising: a furnace shellthat defines a furnace volume, said furnace shell having a centerportion; a reactant gas inlet connected to said furnace shell in fluidcommunication with said furnace volume; a susceptor disposed within saidfurnace shell; an induction coil disposed within said furnace shelladjacent said susceptor, said induction coil comprising a coolingelement; an inlet conduit connected to said furnace shell at said centerportion and through said reactant gas inlet, said inlet conduit being influid communication with said furnace volume; an outlet conduitconnected to said furnace shell in fluid communication with said furnacevolume; a cooling gas supply configured to selectively introduce acooling gas into said furnace volume; and, a blower connected to saidinlet conduit and said outlet conduit in fluid communication therewith,wherein activation of said blower causes cooling gas introduced intosaid furnace volume to flow through said blower, through said inletconduit, over said cooling element, through said outlet conduit, andback to said blower in a closed circuit.
 17. The apparatus of claim 16,wherein said blower comprises a housing and a drive shaft extendingtherefrom, and an inert gas purged dynamic seal between said housing andsaid drive shaft.
 18. In combination, a furnace configured to processrefractory composites, a susceptor disposed within said furnace andconfigured as a cylindrical wall having a center portion disposedbetween two end portions, and an inlet conduit entering said furnace,said center portion having a hole therein with said inlet conduitentering said hole and being configured to introduce cooling gas withinsaid cylindrical wall at said center portion.
 19. The apparatus of claim18, further comprising an insulating bushing disposed within said holemating with said cylindrical wall and said inlet conduit.
 20. Incombination, a furnace configured to process refractory composites, asusceptor disposed within said furnace and configured as a cylindricalwall having a center portion disposed between two end portions, acooling gas inlet conduit entering said furnace, a cooling gas outletconduit exiting said furnace, and a cooling element, said center portionhaving a hole therein with said inlet conduit entering said hole andbeing configured to introduce cooling gas within said cylindrical wallat said center portion, said cooling element being disposed between saidinlet conduit and said outlet conduit so that cooling gas introducedthrough said inlet conduit passes over said cooling element beforeexiting said furnace through said outlet conduit.
 21. The apparatus ofclaim 20, further comprising an induction coil disposed adjacent saidcylindrical wall, said inlet conduit extending through said inductioncoil.
 22. The apparatus of claim 20, further comprising an inductioncoil disposed adjacent said cylindrical wall, said inlet conduitextending through said induction coil, said cooling element beingconfigured to cool said induction coil.
 23. In combination, a furnaceconfigured to process refractory composites, a cooling gas inlet influid communication with said furnace, a cooling gas outlet in fluidcommunication with said furnace, a cooling element disposed within saidfurnace in a location where it may be exposed to cooling gas, and ablower outside said furnace connected to said cooling gas inlet and saidcooling gas outlet, wherein said blower causes cooling gas to flow in aclosed circuit through said cooling gas inlet, through said furnace oversaid cooling element, and through said cooling gas outlet back to saidblower.
 24. The apparatus of claim 23, wherein said blower comprises ahousing and a drive shaft extending therefrom, and an inert gas purgeddynamic seal between said housing and said drive shaft.
 25. A method ofcooling a furnace configured to process refractory composites,comprising the steps of: circulating a cooling gas through said furnace;and, changing the composition of said cooling gas to effect the rate atwhich said furnace is cooled.
 26. The method of claim 25, furthercomprising the step of changing the composition of said cooling gas toproduce a constant rate at which said furnace is cooled.
 27. The methodof claim 25, further comprising the step of altering the flow rate ofcooling gas to effect the rate at which said furnace is cooled.
 28. Themethod of claim 25, further comprising the step of alterating flow rateof cooling gas to produce a constant rate at which said furnace cooled.